The invention relates to an apparatus and a method for inspecting a surface of a sample. In particular the invention relates to an apparatus for inspecting a sample surface using a plurality of charged particle beams, such as a multi-beam scanning electron microscope. The invention may be applied to charged particles of any type, such as electrons, positrons, ions and others.
The invention relates to an apparatus for detecting backscattered charged particles, such as electrons. These backscattered charged particles have almost the same energy as the primary charged particles. It is usually assumed that the charged particles lose up to a few hundred electron volts in the sample before coming out again. The angular distribution of the backscattered charged particles is much larger than the angular distribution of the primary charged particle beams. The backscattered charged particles that travel close to the axis of corresponding primary charged particle beam will travel back through the charged particle optical system along paths that closely resemble the primary charged particles and they will focus in about the same planes as the primary charged particle beams.
Such an apparatus is for example disclosed in U.S. Pat. No. 7,732,762. This US patent discloses an electron microscope comprising an electron emitter which, in use, emits a plurality of primary electron beams which are directed towards a surface of a specimen. The apparatus comprises a first lens system for focusing the electron beams into a first array of separated spots in an intermediate plane between the electron emitter and the specimen surface. The apparatus further comprises a second lens system for directing the primary electron beams from the intermediate plane towards the specimen surface and for focusing all primary electron beams into a second array of individual spots on the specimen surface. In particular the second lens system comprises a cascade of lenses, more in particular a microlens array which comprises an individual microlens for each individual electron beam.
At the intermediate plane, a thin sheet-like plate comprising fluorescent material is disposed. The thin sheet comprises apertures, so that the primary electron beams are allowed to pass. The plate collects the back-scattered electrons and converts the collected electrons into photons. The photons are at least partially directed to an array of photo detectors by means of an optical lens system.
A disadvantage of this system is that the distance between the individual spots on the specimen surface is relatively large due to the use of one objective microlens per electron beam. Consequently the known system is less suitable for inspecting small samples.
It is an object of the present invention to provide a multi charged particle beam apparatus for inspecting a sample surface which provides a new detection arrangement for detecting the back-scattered electrons.